At the southern tip of the Cape Peninsula, perched on a windswept cliff 230 metres above the confluence of the Atlantic and Indian Oceans, sits one of the world's most important atmospheric monitoring stations. The Cape Point Global Atmospheric Watch (GAW) station has been collecting flask measurements of atmospheric CO₂ since 1983 — the longest-running Southern Hemisphere baseline CO₂ record on the African continent.
Operated jointly by the South African Weather Service (SAWS) and NOAA's Global Monitoring Laboratory (GML), the station's isolation from industrial sources and its exposure to clean marine air make the readings exceptionally representative of Southern Hemisphere background air. What those readings show, month by month, is an unbroken climb.
The 40-Year Trend
When the first flask samples were collected in 1983, Cape Point recorded a yearly mean of roughly 341 ppm. By 2023 that figure had climbed to approximately 420 ppm — a rise of nearly 79 ppm over four decades, equivalent to an increase of 23% above the pre-industrial baseline of around 280 ppm.
Annual Mean CO₂ at Cape Point, 1983–2023
Yearly average concentration in parts per million (ppm, dry air). Source: NOAA GML / SAWS Cape Point flask programme.
The rise is relentless but not perfectly linear. Two notable slowdowns stand out:
- 1992–1993 — The 1991 eruption of Mount Pinatubo cooled the planet and appears to have stimulated land-carbon uptake. Growth slowed to under 1 ppm per year in 1992, the quietest stretch in the record.
- 2009 — The Global Financial Crisis temporarily reduced fossil-fuel emissions, producing a brief dip in the annual growth rate.
El Niño years tell the opposite story. In 1998, 2010, and especially 2016, tropical droughts stressed vegetation that would ordinarily absorb carbon, pushing growth rates well above 3 ppm per year.
Milestones: Crossing Each 10 ppm Threshold
| Threshold | Year first crossed | Annual mean that year | Years since previous threshold |
|---|---|---|---|
| 350 ppm | 1988 | 350.9 ppm | — (start of record: 341 ppm) |
| 360 ppm | 1995 | 360.7 ppm | 7 years |
| 370 ppm | 2001 | 370.9 ppm | 6 years |
| 380 ppm | 2006 | 381.1 ppm | 5 years |
| 390 ppm | 2011 | 391.0 ppm | 5 years |
| 400 ppm | 2016 | 403.1 ppm | 5 years |
| 410 ppm | 2019 | 410.2 ppm | 3 years |
| 420 ppm | 2023 | ~420.0 ppm | 4 years |
From 350 to 360 ppm took seven years. From 410 to 420 ppm took four. The pace is accelerating — each successive 10 ppm step is being completed faster than the last.
Monthly Averages: The Seasonal Cycle
Looking at monthly means rather than annual averages reveals a consistent seasonal oscillation. The Southern Hemisphere breathes in a much quieter rhythm than the north — Cape Point shows a peak-to-trough swing of roughly 2 ppm, compared with more than 6 ppm at Mauna Loa, Hawaii.
The reason is geography. The Northern Hemisphere holds most of the world's terrestrial vegetation; its growing season creates a powerful annual drawdown that drives global CO₂ down each NH summer. The Southern Hemisphere, dominated by ocean, lacks that large photosynthetic pulse. The chart below shows the average departure of each calendar month from that year's annual mean — a climatological fingerprint of the seasonal cycle.
Seasonal Cycle: Average Monthly Departure from Annual Mean
Mean deviation of each calendar month from that year's annual mean, averaged across 1983–2023. Purple bars = above annual mean; green bars = below.
CO₂ peaks in July–August, during the Southern Hemisphere winter. At that time the Northern Hemisphere's growing season has not yet pulled concentrations far below their peak, and Southern Ocean winter upwelling releases some stored carbon to the atmosphere. The annual minimum arrives in January–February — Southern summer — when photosynthesis across both hemispheres is at its highest.
Monthly CO₂ per Calendar Month — Long-Run Averages
The table below gives the long-run average CO₂ for each calendar month, computed across the full 1983–2023 record. Because CO₂ has been rising throughout this period, these overall averages cluster around 377 ppm — the mid-point of the 40-year climb. The seasonal swing accounts for about 2 ppm of variation between the highest month (July) and the lowest (February).
| Month | Long-run avg (ppm) | Departure from annual mean | Season (Southern Hemisphere) |
|---|---|---|---|
| January | 376.5 | −0.8 ppm | Midsummer |
| February | 376.3 | −1.0 ppm ▼ lowest | Late summer |
| March | 376.7 | −0.6 ppm | Early autumn |
| April | 377.2 | −0.1 ppm | Autumn |
| May | 377.7 | +0.4 ppm | Late autumn |
| June | 378.0 | +0.7 ppm | Early winter |
| July | 378.3 | +1.0 ppm ▲ highest | Midwinter |
| August | 378.2 | +0.9 ppm | Late winter |
| September | 377.9 | +0.6 ppm | Early spring |
| October | 377.4 | +0.1 ppm | Spring |
| November | 377.0 | −0.3 ppm | Late spring |
| December | 376.7 | −0.6 ppm | Early summer |
Recent Years in Detail (2019–2023)
Plotting monthly values for the past five years puts both the trend and the seasonal cycle in sharp relief. Each year's curve sits about 2–3 ppm above the previous one, and the gentle seasonal wave — peak in winter, trough in summer — repeats faithfully each year.
Monthly CO₂ at Cape Point, 2019–2023
Monthly mean CO₂ (ppm) for each year shown as a separate line. Hover or tap for exact values.
Why Cape Point Matters
Most people encounter the CO₂ record through Mauna Loa Observatory, which has been measured continuously since 1958. Cape Point plays a vital complementary role: it demonstrates that the CO₂ rise is a truly global phenomenon, not an artefact of measurements taken close to Northern Hemisphere industry or in the shadow of Hawaiian volcanic outgassing. Cape Point's numbers track Mauna Loa to within 1–2 ppm, confirming that CO₂ mixes freely across hemispheres on timescales of months to a year.
The station also sits at the edge of the Southern Ocean — the largest single carbon sink on Earth. Shifts in how effectively the ocean absorbs CO₂ have the potential to appear first in Cape Point's baseline readings before they propagate into global averages. That makes these 40 years of data not merely a historical ledger, but an early-warning system for one of Earth's most critical climate feedbacks.
At the current growth rate of roughly 2.4 ppm per year, Cape Point will record its first 430 ppm annual mean as early as 2028, and could reach 450 ppm before 2040 — levels not seen on Earth since the Pliocene epoch, approximately 3 million years ago.
Data source: NOAA Global Monitoring Laboratory — Cape Point (CPT), in partnership with the South African Weather Service (SAWS). Monthly flask CO₂ measurements from the CCGG cooperative air sampling network. Values rounded to one decimal place; annual means derived from monthly flask samples. Seasonal climatology computed as the average departure of each calendar month from its year's annual mean across the full 1983–2023 record. Long-run monthly averages are based on a record-wide grand mean of approximately 377.3 ppm.